Welcome Cheri2812,
We understand that you're having issues in the Mac Mail app on your iMac. We'll do our best to assist with this.
If you haven't already, we'd check the following settings: Change preferences in Mail on Mac - Apple Support and Change Account Information preferences in Mail on Mac - Apple Support
Download Attachments
Mail always downloads media attachments (such as image, PDF, video, or audio files) that you receive.
To have Mail automatically download other types of attachments (such as spreadsheets or zipped files), click the pop-up menu, then choose an option:
- All: Mail always downloads these attachments.
- Recent: Mail downloads only attachments of this type received within the past 15 months.
- None: Mail doesn’t download any of these attachments.
Mail downloads attachments you receive to a special folder in your home Library folder.
When you have 'All' selected, does it download these attachments without this error? If not, then it would be best to contact Apple Support
Hoping to hear how it goes, take care.